(1) Field of the Invention
The present invention relates to a melt-moldable low-melting polyimide copolymer and a method for preparing the same.
(2) Description of the Prior Art
In general, polyimides are widely used as protective materials and insulating materials in electronic equipment fields, and adhesive materials, films and structural materials because they are organic polymer compounds having high heat resistance, good electrical characteristics and mechanical characteristics. However, conventional polyimides have high melting points or glass transition temperatures, which are in excess of or in the vicinity of thermal decomposition temperatures of most polyimides, and thus it is usually difficult to melt-mold the polyimides. For this reason, the molding of the polyimide has been performed almost by molding techniques for thermosetting resins, for example, by coating and calcining a precursor or a soluble polyimide solution of the polyimide, or by compression-molding a powdery polyimide at high temperature and pressure. If a polyimide is obtained to which molding techniques for thermoplastic resins such as extrusion and injection molding can be applied, the improvement of productivity as well as the molding of articles having various shapes will become possible, which will heighten economical effect. Therefore, suggestions regarding meltable polyimides have often been made. For example, in Japanese Patent Laid-open Publication No. 203132/1986, there is suggested a crystalline silicone imide copolymer. In addition, Japanese Patent Laid-open Publication No. 250031/1986 reveals a specific pyromellitic imide copolymer as the meltable polyimide copolymer.
However, with regard to the copolymers referred to in the above-mentioned Japanese Patent Laid-open Publication No. 203132/1986, their melting points are in the range of 140 to 210.degree. C. judging from values in its examples. In this laid-open invention, it can be considered that heat resistance which is one feature of the polyimide is sacrificed in order to improve moldability. In Japanese Patent Laid-open Publication No. 250031/1986, it is described that the suggested copolymer can be molded into a film by melt-press at 400.degree. C., and its effect is also shown. However, any melting point of the copolymer is not mentioned therein, and it is not apparent either whether or not the copolymer is, in fact, the crystalline polymer.
In general, even the polyimides each comprising an aromatic skeleton as the main component are partially decomposed at a temperature higher than 450.degree. C., so that articles molded therefrom will be noticeably colored. In view of the easiness of molding, a practical temperature in the extrusion and the injection molding is usually about 50.degree. C. above the melting point of the polyimide. Thus, it is preferable that the polyimide has a melting point of 400.degree. C. or less, and additionally in view of use applications where heat resistance is required, the polyimide preferably has a melting point of 300.degree. C. or more. As understood from the foregoing, the polyimides having melting points of 300.degree. to 400.degree. C. are very valuable, and thus the development of such polyimides has been demanded.